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To date, numerous materials, including various quantum dots and dyes, have been widely used for the ultrasensitive detection of toxic metal ions and as security inks to hide information. Nevertheless, because of the poor dispersibility of solid-state materials, security inks based on such materials have been scarcely reported. Herein, a highly dispersible and water-stable metal−organic framework (MOF; NH 2 -MIL-125(Ti)) is used as an invisible security ink for data coding, encryption, and decryption via its "turn-on/off" switching by treatment with ethylenediaminetetraacetic acid and Pb 2+ . Notably, the concentration of the Pb 2+ solution used to turn off the fluorescence of the MOF was lower than the limit established by several regulatory agencies for drinking water. The MOF was also used as a sensitive probe for the rapid and ultrasensitive detection of Pb 2+ ions at a concentration of 7.7 pM which is one of the lowest detection limits reported for such a system. The MOF also shows high selectivity for various transition metal ions that can competitively bound on the ligand. Analyses using Fourier transform infrared spectroscopy,X-ray photoelectron, and UV photoemission spectroscopy clearly revealed the roles of the surface functional groups and the mechanism of the "on/off" switching behavior of the MOF.

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Recently, active bubble-propelled micromotors have attracted great attention for fuel applications. However, for generating bubble-propelled micromotors, additional catalysts, such as Pt, Ag, and Ru, are required. These catalysts are expensive, toxic, and highly unstable for broad applications. To overcome these issues, in this study, we present an innovative methodology for the preparation of self-propelled motor machines using naturally occurring diatom frustules. This natural diatom motor shows effective motion in the presence of a very low concentration (0.8%) of H2O2 as a fuel at pH 7. Due to the unique 3D anisotropic shape of the diatom, the self-propelled motor exhibited unidirectional motion with a speed of 50 μm s-1 and followed pseudo first-order kinetics. It was found that a trace amount of iron oxide (Fe2O3) in the diatom was converted into Fe3O4, which can act as a catalyst to achieve the facile decomposition of H2O2. Interestingly, "braking" of the unidirectional motion was observed upon treatment with EDTA, which blocked the catalytically active site. These results illustrate that diatom catalytic micromotors have opened a new era in the field of catalysis and bioengineering applications.

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The imperfection instability, recyclability, and separation factors of metal−organic frameworks (MOFs) limit their practical applications in the field of catalysis and water purification. Designing MOFs that are benign, flexible, and separable is still a critical challenge. Up to now, most of MOFs have been coated with conventional synthetic polymers, which are undegradable and carcinogenic. However, no studies have reported the stepwise growth of biocompatible polymer-capped Fe 3 O 4 (PFe 3 O 4 ) nanoparticles (NPs) onto the NH 2 -MIL-125 (Ti) surface (ternary composite). In this study, a simple stepwise embedding of PFe 3 O 4 NPs onto NH 2 -MIL-125 (Ti) was successfully employed and used for efficient aquatic scavenging, which can allow synergetic cooperative adsorption with functionality on both the biopolymer and MOF surface. The obtained transmission electron microscopy (TEM) and high-resolution TEM images illustrate that the PFe 3 O 4 NPs were uniformly embedded onto the surface of the MOF. The composite was employed for the quick and significant removal of Pb(II) from aqueous solution. The effects of various parameters like the pH, contact time, initial metal-ion concentration, interfering ions, and temperature on the adsorption capacity of the nanoporous composite were examined. The Langmuir model presented the best fitting with a maximum adsorption capacity of 561.7 mg g −1 at pH 5 and 298 K. Moreover, increasing the PFe 3 O 4 precursor on nanoporous NH 2 -MIL-125 (Ti) decreased the recovery time (21 s) and enhanced the adsorption process because the same MOF can be recycled six times without obvious loss of the adsorption capacity of Pb(II) in water. Therefore, we can finally conclude that, due to the coating of Fe 3 O 4 with a biopolymer, the composite showed not only highly efficiency in metal-ion adsorption but also high stability for recycling of the material, which is significant for its future practical use in the treatment of industrial waste discharge.